The invention relates to a noise reducing apparatus comprising resonators or so-called silators.
German Patent Publication (DE-OS) No. 2,632,290.7 discloses a basic construction of so-called silators which may be defined as vibrating resonators changing their volume and thereby having a silencing effect. These silators comprise two lentil shaped, vaulted sheet metal halves interconnected in a vacuum tight manner. The space formed inside the vaulting is evacuated. Such silators have a resonance frequency which may be adjusted substantially by selecting the wall thickness, the vaulting height, and the diameter of the lentil shaped silators.
So-called Helmholtz resonators are also well known in the art for noise reduction purposes. However, Helmholtz resonators require, as compared to silators, a substantially larger volume and surface area which limits the use of such Helmholtz resonators.